Restoration of images blurred due to the point spread function of the lens of a digital camera is an important problem in signal processing. Conventional image restoration algorithms exploit known point spread function information to sharpen the blurred image. Typically, the lens point spread function is derived from the modulation transfer function (MTF) or the optical transfer function (OTF) of the lens. In the absence of any knowledge of the MTF or OTF, the point spread function can be estimated from the blurred image itself. This approach is known as blind deblurring. Therefore, point spread function estimation is a key step in image restoration algorithms.
A parametric form for the lens MTF is highly desirable as it can be implemented efficiently in the memory constraint hardware architecture of the digital cameras. A closed-form equation for the diffraction-limited lens modulation transfer function is well known. However, it is extremely complicated to develop a closed-form equation for the MTF of a real zoom lens system, particularly a zoom lens having a combination of several lens elements where the MTF will be influenced by several optical parameters including focal length, F-number (F/#) and focus distance. Therefore, it is generally necessary to rely on measurement based methods, or computer models that can predict the lens characteristics.
Sean et al., in U.S. Pat. No. 6,628,329, entitled “Correction of Position Dependent Blur in a Digital Image,” describes a method for calibration of MTF correction. The method involves capturing a target with many MTF features (e.g., slanted edges), and computes a grid of spatially varying sharpening kernels to compensate. However, the requirement to store a plurality of sharpening kernels is memory intensive and cannot easily be extended to zoom lens configurations.
In U.S. Patent Application Publication 2007/0036461, entitled “Correction Deconvolution of a Digital Image”, Timothy describes a method to restore a defocused image using a parametric equation for the OTF of a diffraction-limited lens. However, the method is only applicable to fixed-focal length diffraction limited lenses.
Naoki et al., in U.S. Pat. No. 6,356,304, entitled “Method for Processing Video Signal and Apparatus for Processing Video Signal,” describes a method for controlling the aperture of a digital camera, and controls sharpening gain depending on the aperture setting.
European Patent Application Publication EP1954030A1 to Hatori et al., entitled “Image Pickup Apparatus and Image Processing Method,” discloses selecting a deblurring filter from a stored set of deblurring filters based on focal length and subject distance.
Thus, there exists a need for an efficient method for characterizing the MTF of a zoom lens which can subsequently be used to sharpen blurred images captured under a variety of zoom and aperture conditions.